CALibrate has an easy to use and intuitive permission engine.

Calendar permissions can be set for groups of people (depending on the type of calendar) or individuals. The meaning of the permissions are as follows:

append -- person or persons may append events to the calendar
delete - person or persons may delete events from the calendar
meta - person or persons can do anything to a calendar, including delete it, change its name, etc.
modify - person or persons may edit (change/update) events on the calendar
subscribe - person or persons may ready only the calendar